#include <string.h>
#include <stdint.h>

#define aliases __attribute__((__may_alias__))
#define byte_repeat(x) ((size_t)~0 / 0xff * (x))
#define word_has_zero(x) (((x) - byte_repeat(0x01)) & ~(x) & byte_repeat(0x80))

void *memchr(const void *_s, int i, size_t n)
{
	i = (unsigned char)i;
	const unsigned char *s = _s;
	const size_t aliases *ws, wi = byte_repeat(i);

	if (n < sizeof(size_t) * 3) goto bytewise;

	for (; (uintptr_t)s & sizeof(size_t) - 1 && *s != i; s++, n--);
	if ((uintptr_t)s & sizeof(size_t) - 1) return (void *)s;

	ws = (const void *)s;
	for (; n >= sizeof(size_t) && !word_has_zero(*ws ^ wi)
	     ; ws++, n -= sizeof(size_t));
	s = (const void *)ws;

bytewise:
	for (; n && *s != i; s++, n--);

	return n ? (void *)s : 0;
}